Dwarves freaking out on the walls during zombie invasion

In the newer versions (specifically 0.31.16 which I'm currently playing), you can specify an individual Dwarf's equipment by type, or even down to the specific item. From the (current at the time of writing) Dwarf Fortress wiki:

In the military screen, press n to open the uniforms tab. Each listing under the 'Uniforms' header is essentially a pre-designed set of equipment that you can quickly apply to any individual soldier or entire squad, much like a template. You can create new uniforms if you want and add or remove items from any uniform set by navigating this menu. Uniform templates are only created in this tab, not applied.

While still in the military screen, press e to open the equipment tab. The default sub-tab, 'View/Customize will be open. In this screen you can select individual dwarves and apply individual pieces of equipment to them, from Armor, Leggings, Helms, Gloves, Boots, SShields, and Weapons, as well as Material and Color depending on the piece of equipment highlighted. To select a specific piece of equipment (such as an artifact), select 'specific _____' under that equipment type (eg. 'specific armor' in the Armor field); for your convenience, highest-quality equipment is listed first.

As of the current version of Dwarf Fortress, Dwarves and Adventurers do not stack things. Ever.

If the bolts are metal, it might be better to melt them down instead so that you will eventually recoup some of the metal for a new stack, because the alternative, as you yourself mention, is a dwarf running around with a 1 stack of ammunition.